1. Field of the Invention
The present invention relates to a switching power circuit and a control method for the switching power circuit and more particularly relates to the switching power circuit for controlling a harmonic current of an AC (Alternating Current) power such as a commercial power and an external form of the switching power circuit is made small.
The present application claims priority of Japanese Patent Application No. 2000-073177 filed on Mar. 15, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
A condenser-input switching power circuit rectifies an AC (Alternating Current) power such as a commercial power and smoothes the AC power with a condenser, and then converts the AC power into a predetermined DC voltage level with a DCxe2x80x94DC converter so as to keep an output voltage constant by a negative feedback control regardless of changes of an input voltage, a load and a like.
A conventional switching power circuit, as shown in FIG. 4, is provided with a rectification circuit 1, a smoothing condenser 2, a switching circuit 3, a transformer 4, a rectification circuit 5, a smoothing condenser 6, a voltage change detecting circuit 7 and a control circuit 8.
The rectification circuit 1 executes a full-wave (or half-wave) rectification to an AC power xe2x80x9cinxe2x80x9d such as a commercial power and outputs a first pulsating voltage.
The smoothing condenser 2 is an aluminum electrolytic condenser, and smoothes the first pulsating voltage so as to output a DC voltage V2.
The switching circuit 3 controls ON/OFF of the DC voltage V2 based on a given control signal CTA and outputs an AC voltage V3 having a pulse width corresponding to the control signal CTA at a predetermined frequency f (such as 20 kHz to 500 kHz).
The transformer 4 transforms the AC voltage V3 to a predetermined voltage, namely, an AC voltage V4.
The rectification circuit 5 rectifies the AC voltage V4 to a pulsating voltage V5.
The smoothing condenser 6 is an aluminum electrolytic condenser, and smoothes the pulsating voltage V5 so as to output a DC voltage V6 to a load not shown.
The voltage change detecting circuit 7 detects a change of the DC voltage V6 and outputs a detection signal V7.
The control circuit 8 gives the control signal CTA having a pulse width corresponding to the detection signal V7 to the switching circuit 3 at a predetermined frequency f.
In the switching power circuit, the AC voltage xe2x80x9cinxe2x80x9d is full-wave rectified in the rectification circuit 1, the first pulsating voltage is output and smoothed in the smoothing condenser 2, and then the DC voltage V2 is output. The DC voltage V2 is ON/OFF controlled in the switching circuit 3 based on the control signal CTA, and the AC voltage V3 of the pulse width corresponding to the control signal CTA at the predetermined frequency f is output.
The AC voltage V3 is transformed to the AC voltage V4 in the transformer 4, the AC voltage V4 is rectified in the rectification circuit 5 and the pulsating voltage V5 is output. The pulsating voltage V5 is smoothed in the smoothing condenser 6 and the DC voltage V6 is supplied to the load. Also, the change of the DC voltage V6 is detected in the voltage change detecting circuit 7 and the detection signal V7 is output. The detection signal V7 is input to the control circuit 8. When the level of the detection signal V7 is lower than a reference value, the pulse width of the control signal CTA is controlled so that a time of an ON state in the switching circuit 3 becomes long, and thereby the negative feedback control is executed so that the DC voltage V6 becomes constant. Also, when the level of the detection signal V7 is higher than the reference value, the pulse width of the control signal CTA is controlled so that the time of the ON state in the switching circuit 3 becomes short, and thereby the negative feedback control is executed so that the DC voltage V6 becomes constant.
However, there are following problems in the conventional switching power circuit.
FIG. 5A is a graph showing a voltage waveform Va of the AC power xe2x80x9cinxe2x80x9d and FIG. 5B is a graph showing a current waveform Ia of the AC power xe2x80x9cinxe2x80x9d. In FIG. 5A, an axis of ordinates indicates a voltage value V and an axis of abscissas indicates a time t. In FIG. 5B, an axis of ordinates indicates a current value I and an axis of abscissas indicates a time t.
In the conventional switching power circuit, as shown in FIG. 5A and FIG. 5B, a large pulse current flows instantaneously near a peak of the voltage waveform (sine wave) Va caused by a peak of the current waveform Ia and an harmonic current flows at a side of the AC power xe2x80x9cinxe2x80x9d. The harmonic current produces a harmful influence on the AC power xe2x80x9cinxe2x80x9d, and therefore, there are problems in that a fault occurs in another electronic apparatus connected to the AC power xe2x80x9cinxe2x80x9d and a transmission loss becomes large. For example, when a video apparatus and an audio apparatus are commonly connected to the AC power xe2x80x9cinxe2x80x9d to which the switching power circuit is connected, there are harmful influences in that an image quality of the video apparatus and a tone quality of the audio apparatus deteriorate caused by the harmonic current. Therefore, a restriction is determined against the harmonic voltage at present.
Also, the switching power circuit is provided with the smoothing condenser 2 and the smoothing condenser 6 which are aluminum electrolytic condensers of comparatively large sizes, and therefore it is difficult to make an outside form of the switching power circuit small. Therefore, there is a problem in that the switching power circuit can not be installed in a small-size apparatus.
To solve these problems, a switching power circuit is proposed in which measures are taken against the harmonic voltage with an unique circuit configuration. The switching power circuit is discussed in xe2x80x9cpower harmonic measure technique and design examplexe2x80x9d, written by Morio Sato, on page 321 of Transistor Technique (CQ Publisher), April 1998.
FIG. 6 is a circuit diagram showing an example of the switching power circuit discussed in this paper (Transistor Technique).
The switching power circuit, as shown in FIG. 6, is provided with a rectification circuit 11, a coil 12, a diode 13, a condenser 14, a condenser 15, an N-channel MOSFET (hereafter, called an NMOS) 16, a transformer 17, a diode 18, a coil 19, a diode 20 and a condenser 21. The condenser 15 and the condenser 20 are aluminum electrolytic condensers.
In the switching power circuit, when the NMOS 16 becomes OFF, a current indicated by a current rout L flows by a fly-back voltage which generates in a first coil 17a of the transformer 17, and the condenser 14 is charged. Then, when the NMOS 16 is turned ON, the condenser 14 is discharged. Since a discharge current rout M always passes through the AC power, the AC current is forcibly taken. As a result, a current flows also in a section in which a voltage between output ends of the rectification circuit 11 is lower than a voltage of the condenser 15. Therefore, a current flows also in a section though no current flows in the condenser-input switching power circuit, an conducting angle (namely, a period in which an AC current flows) enlarges and no harmonic current flows to the AC power xe2x80x9cinxe2x80x9d. Further, since the coil 12 is excited by a current indicated by the discharge current rout M of the condenser 14, excited energy is discharged as a current indicated by a current rout N and the AC current is forcibly taken. Then, the excited energy moves from the condenser 14 to the coil 12 and the condenser 15. Then, the voltage of the condenser 15 is ON/OFF controlled by the NMOS 16, and an operation approximately similar to that of the switching power circuit shown in FIG. 4 is executed.
In the switching power circuit, the problem of the harmonic current at the side of the AC power xe2x80x9cinxe2x80x9d is solved. However, there are problems in that it is difficult to make an outside form small similarly to the switching power circuit shown in FIG. 4 and it is impossible to install the switching power circuit in a small apparatus, since parts are many in comparison with the switching power circuit shown in FIG. 4 and the condenser 15 and the condenser 21 are aluminum electrolytic condensers. In addition to the switching power circuit, though makers develop switching power circuits in which harmonic measures are taken by unique circuit configurations, those circuits have a problem in that circuits become complex and parts increase.
In view of the above, it is an object of the present invention to provide a switching power circuit and a control method for the switching power circuit which controls a harmonic current of an AC power and which is small in size.
According to a first aspect of the present invention, there is provided a switching power circuit including:
a first rectification circuit for rectifying an AC (Alternating Current) power so as to generate a first pulsating voltage;
a switching circuit for controlling ON/OFF of the first pulsating voltage based on a control signal which is input so as to generate a first AC voltage having a predetermined frequency higher than a frequency of the AC power and having a pulse width corresponding to the control signal;
a transforming circuit for transforming the first AC voltage so as to generate a second AC voltage of a predetermined voltage value;
a second rectification circuit for rectifying the second AC voltage so as to generate a second pulsating voltage;
a smoothing circuit for smoothing the second pulsating voltage so as to generate a DC (Direct Current) voltage and for applying the DC voltage to a load;
a voltage change detecting circuit for detecting a change of the DC voltage in a period longer than a period of a ripple included in the DC voltage so as to generate a detection signal; and
a control circuit for generating the control signal used to execute a negative feed-back control of the pulse width of the first AC voltage based on a level of the detection signal.
In the foregoing, a preferable mode is one wherein the smoothing circuit is a condenser of which a volume per unit capacity is smaller than that of an aluminum electrolytic condenser.
Also, a preferable mode is one wherein the smoothing circuit is an electrical double layer condenser of which a volume per unit capacity is smaller than that of the aluminum electrolytic condenser.
Furthermore, a preferable mode is one wherein the voltage change detecting circuit includes:
a reference voltage comparing section for comparing a level of the DC voltage with a reference voltage corresponding to a set value of the DC voltage and for outputting a comparison result signal; and
a low-pass filter for receiving the comparison result signal and for passing only frequency components lower than the frequency of the ripple included in the DC voltage in the comparison result signal so as to output the frequency components as the detection signal.
According to a second aspect of the present invention, there is provided a control method for a switching power circuit including a first rectification circuit for rectifying an AC (Alternating Current) power so as to generate a first pulsating voltage, a switching circuit for controlling ON/OFF of the first pulsating voltage based on a control signal which is input so as to generate a first AC voltage having a predetermined frequency higher than a frequency of the AC power and having a pulse width corresponding to the control signal, a transforming circuit for transforming the first AC voltage so as to generate a second AC voltage of a predetermined voltage value, a second rectification circuit for rectifying the second AC voltage so as to generate a second pulsating voltage and a smoothing circuit for smoothing the second pulsating voltage so as to generate a DC (Direct Current) voltage and for applying the DC voltage to a load, the control method including:
a voltage change detecting process of detecting a change of the DC voltage in a period longer than a period of a ripple included in the DC voltage so as to generate a detection signal; and
a control signal generating process of generating the control signal used to execute a negative feed-back control for the pulse width of the first AC voltage based on a level of the detection signal.
In the foregoing, a preferable mode is one where in the voltage change detecting process includes:
a reference voltage comparing process of comparing a level of the DC voltage with a reference voltage corresponding to a set value of the DC voltage and of outputting a comparison result signal; and
a low-pass process of passing only frequency components lower than the frequency of the ripple included in the DC voltage in the comparison result signal so as to output the frequency components as the detection signal.
With this configuration, since there is no smoothing condenser at a rear step of the first rectification circuit, it is possible to control a harmonic current flowing to the AC power with a simple configuration and it is possible to make an outside form of the switching power circuit small. Therefore, when a video apparatus, an audio apparatus, and a like are commonly connected to the AC power to which the switching power circuit is connected, it is possible to avoid deterioration of an image quality of the video apparatus and a tone quality of the audio apparatus. Also, since the smoothing circuit is an electronic double layer condenser, it is possible to make the switching power circuit smaller than a conventional switching power circuit using an aluminum electrolytic condenser, and therefore it is possible to install the switching power circuit into a smaller apparatus. Furthermore, since the voltage change detecting circuit is not responsive to the ripple components included in the DC voltage, a change of the DC voltage of a frequency lower than a frequency of the ripple included in the DC voltage is detected and an output current supplied to the load can be proportional to the DC voltage.